Recent technological developments are expected to establish the automated closed-loop control of physiological variables as a potentially powerful tool for physiological research and clinical therapy. In spite of this potential, quantitative understanding of the performance of such control systems in realistic laboratory or clinical situations is lacking, and the possibility of using a combination of parameter identification and automatic control for obtaining new physiological information has been hardly explored. The objective of this project is to gain a basic understanding of applying modern control theory to the characterization and manipulation of physiological systems. In order to concentrate on the control problem, and to facilitate the effective utilization of mathematical models, the automated control of arterial blood pressure will be chosen as the test system. Specifically, chronically catheterized and instrumented dogs will be subjected to automated blood pressure control using vasoactive drugs. A mathematical model of the physiological system will be established, and the control will be administered on the basis of periodic identification of model parameters. The performance of manual, automated algorithmic, and automated optimal control of blood pressure will be compared using objective quantitative measures of the performance. The work is expected to improve a presently incomplete understanding of the dynamics of a physiological system of considerable importance, establish the feasibility of continuous on-line control and parameter estimation, and provide a clear-cut comparison of optimal control with traditional means of controlling physiological systems.